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The pachycephalosaurian squamosal is one of the most diagnostic bones in this enigmatic group of dinosaurs, but little is known about variation in its morphology. Despite this, features of squamosal morphology are often used in diagnoses and phylogenetic studies. The recently proposed hypothesis of an ontogenetic transition from Dracorex to Pachycephalosaurus implies a large amount of ontogenetic variation in squamosal ornamentation that has not been well documented due to the small number of squamosals for these taxa. Stegoceras validum provides an important model for examining variation in squamosal morphology due to the large number of squamosals available. Here we examine the morphology of 14 S. validum squamosals both qualitatively and quantitatively. We find that although both node number and shape vary considerably, there is no clear ontogenetic pattern, which suggests a high degree of individual variation. The ornamental pattern, however, is maintained throughout the sample. Increase in n...

Colepiocephale lambei from the Foremost Formation of Alberta is a problematic pachycephalosaurid that has recently been hypothesized as (1) synonymous with Stegoceras validum, (2) a distinct species within the Stegoceras clade, and (3) an unusual taxon more derived than Stegoceras. Conflict among these hypotheses arises from different interpretations of the parietosquamosal region of the dome. Re-evaluation of the morphology of C. lambei based on all known material from Alberta indicates that the posteromedial extension of the parietal is broken off at its base in every specimen, and what was interpreted as the posterior margin of the dome actually is the anterior margins of large supratemporal fenestrae. The squamosals are not preserved in any specimen, but the form of the parietal suggests that they were present in the typical pachycephalosaurid configuration. Colepiocephale lambei is considered to be a distinct taxon based on several autapomorphies and stratigraphic segregation from S. validum...